This invention relates generally to a method for managing the operation of a fuel processor. More specifically, this invention relates to a method to inject liquid water, in combination with superheated steam and compressed air into the primary reactor of a fuel processor to support transient, startup and semi-continuous operation of the fuel processor.
Hydrogen is used as a fuel in many applications today, including in fuel cells producing electric power. Fuel cells have also been proposed for use in electrical vehicular power plants to replace internal combustion engines.
In certain fuel cells, hydrogen (H2) is the anode reactant, i.e., the fuel, and oxygen is the cathode reactant, i.e., the oxidant. The oxygen can be supplied in either a pure form (O2), or as air (a mixture primarily containing O2 and N2). The hydrogen is typically provided by dissociating a hydrogen rich fuel.
For vehicular applications, it is desirable to use a liquid fuel such as methanol, gasoline, diesel, or the like, as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store on board and there is a nationwide infracture for supplying liquid fuels. Such fuels must first be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within a fuel processor, typically in several steps, including a fuel/air reaction (exothermic), and a fuel/water (as steam) reforming step (endothermic). Both these reactions often occur within the same component, known as an autothermal reformer, and herein as a primary reactor. The primary reactor typically yields a reformate gas comprising primarily of hydrogen and carbon monoxide. The carbon monoxide may then be further reacted with water in a water gas shift reactor and air in a preferential oxidation reactor to produce a reformate stream of sufficiently high quality for use in a proton exchange membrane (PEM) fuel cell. Output levels from the primary reactor directly impact all further downstream components. The optimum operating conditions for the primary reactor are dependent on the inlet temperature and steam to carbon ratio (S/C).
A general description of the primary reactor and water gas shift reactor, and the impact of water on their performance, are included below:
CxHy+O2+H2Oxe2x86x92H2+CO+CO2+H2O+CH4xe2x80x83xe2x80x83(1) 
CO+H2O⇄CO2+H2xe2x80x83xe2x80x83(2) 
CO+H2⇄CH4+H2Oxe2x80x83xe2x80x83(3) 
As shown in equation (1), water is used as a source of oxygen (along with the air) for the incomplete oxidation of a hydrocarbon fuel. Water can also be found as a reactant and product in two secondary equilibrium reactions that can occur within the primary reactor. In the first of these secondary reactions (2), when water is used as a reactant (and is consumed within the reaction), decreasing its concentration (lowering the S/C ratio), will slow down the forward reaction, resulting in higher CO concentrations in the primary reactor exhaust. In the second of these secondary reactions (3), when water results as a product, decreasing its concentration (lowering the S/C ratio) will speed up the forward reaction resulting in higher methane (CH4) concentrations in the primary reactor exhaust. Higher methane concentrations can be attributed to losses in efficiency with respect to primary reactor operation.
In a downstream water gas shift reactor (WGSR), steam is used for oxidation of carbon monoxide (CO). Water is used as a source of oxygen for the complete oxidation of CO in the primary reactor exhaust which feeds the WGSR. For the forward reaction, water is used as a reactant. Decreasing its concentration (lowering the S/C ratio) will slow down the forward reaction resulting in higher CO concentrations in the WGSR exhaust. All CO that exits the WGSR is normally fed to a preferential oxidation reactor (PrOx) wherein any remaining CO is oxidized to form CO2. Any CO conversion in the PrOx is less efficient than in the WGSR because hydrogen is a product of CO oxidation in the WGSR, while hydrogen is consumed in the PrOx. Higher exiting CO concentrations can be attributed to losses in efficiency with respect to WGSR operation.
In order to increase efficiency, fuel processors often use heat generated within the process to heat other process streams. In many cases, steam can be generated and superheated by a number of different process streams within a fuel processor sub-system. During transient operations (i.e., where the power level is changing including startup and shutdown) an imbalance in the amount of heat needed with respect to the amount of heat available could be severe enough to create temporary deficiencies in the steam flow entering the primary reactor, where the initial breakdown of the fuel occurs. Steam deficiencies can cause disruptions in the expected product profile exiting the reactor which could further disrupt the balance of the system.
Superheated steam and air are together used to dissociate the raw fuel source in the primary reactor of a fuel processor to produce a reformate gas in a first step in producing the desired hydrogen output product. The steam is normally produced in a vaporizer. Normal variances in steam demand can be supplied by changing the water supply rate and/or heat input to the vaporizer. A deficiency exists, however, in that the vaporizer has a lag time between its steady state output of steam and its output following a system transient. This is due to the material and mass of the vaporizer, as well as its design efficiency of steam production for a given heat input.
During a transient event, the volume of steam in the primary reactor may lag the total volume necessary to maintain an optimum steam to carbon (S/C) ratio, due to a temporary heat imbalance between the existing and new operating levels, largely corresponding to the normal lag of the vaporizer. Accordingly, the vaporizer steam volume rate of change currently limits the rate of change of hydrogen production from the fuel processor during a transient. A method to enhance steam volume available in the primary reactor during transient and startup operation is therefore desirable.
A further system drawback exists during a startup phase. During this phase, system components must be preheated to normal operating temperature. Due to limits of heatup rate and total heat that can be withdrawn from a partially operating system, steam volume supplied by the vaporizer to the primary reactor may also be insufficient. An alternate source of water to supplement the steam for operation of the primary reactor is therefore desirable during the startup phase.
In addition to the above limitations, changing the volume of air supplied to the primary reactor produces differences in the reaction rate, and therefore the heat load developed by this exothermic (partial oxidation) reaction. Oxygen, normally provided by the air flow, is largely consumed in the initial or upstream stage of the primary reactor. Adjusting the air flow rate to the primary reactor at a time when steam volume in the primary reactor is reduced, or when the system is heating up, effects an oxygen to carbon (O/C) ratio. Up to an optimum point, increasing the air flow rate into the primary reactor can react an increased volume of fuel for a given volume of available steam. A method to control the air flow rate, separately or in concert with the steam (or liquid water) flow rate into the primary reactor, is therefore desirable during startup and transient operation of a fuel processor.
A method for managing the operation of a fuel processor to support transient, startup and semi-continuous operations is therefore desirable.
The above deficiencies and drawbacks are overcome by the method of the present invention. The preferred embodiment of the invention provides for injection of liquid water into the normal stream of steam entering the primary reactor of a fuel processor. Liquid water is injected into the steam supply line to overcome a temporary deficit of steam supplied by the vaporizer to the primary reactor. During up-power transients, the volume of steam entering the primary reactor may lag the total volume necessary. To correct the lag in total steam volume during this transient, and therefore to maintain a constant S/C ratio, liquid water from a local water source is injected directly into the steam supply line.
The primary reactor inlet steam temperature is usually in the superheated steam temperature range, and would therefore contain sufficient thermal energy to vaporize and superheat the anticipated volume of liquid water to be injected. The liquid water can be injected in a variety of forms, exemplary forms including water stream, water spray such as through an orifice, or as atomized particles. Liquid water injection provides instantaneous control of the steam volumetric flow, and thus control of the S/C ratio of the primary reactor during those times where steam deficits exist in the system.
In order to control the supplemental water flow rate, knowledge of the desired steam flow rate as well as the steam flow exiting the vaporizer is required. The desired steam flow rate is a constraint of the system in order to maintain a constant S/C ratio. The desired S/C ratio is based on the system design which takes into account many predetermined factors including system pressure, primary reactor operating temperature, water gas shift reactor sizing, fuel properties, humidification requirements, and others. Assuming the predetermined factors are given, the only input required to control the supplemental water flow is the steam flow exiting the vaporizer.
In the preferred version of the invention, a method of operating a primary reactor in a fuel processing system is provided, the method comprising the steps of generating a superheated steam stream; selectively injecting a water stream into said superheated steam stream to form a steam stream mixture; introducing said steam stream mixture to an inlet of a primary reactor; providing a hydrocarbon fuel stream to said inlet of said primary reactor; connecting an air stream to said inlet of said primary reactor; dissociating said steam stream, said fuel stream and said air stream in said primary reactor to form a hydrogen-containing reformate, wherein said water stream is selectively injected into said superheated steam stream based on an operating state of said primary reactor.
When liquid water is added to a stream containing vaporized or superheated water, the temperature of the stream will be lowered due to the sensible heat and latent heat of vaporization of the supplemental water. For a given S/C ratio, there exists a relationship between the inlet mixture temperature and the O/C ratio required to maintain optimum efficiency. Compensation for the inlet mixture temperature decrease can be made by increasing the air flow rate (increasing the O/C ratio) in the inlet flow stream to the primary reactor to maintain a constant outlet temperature. For a given S/C ratio, the outlet temperature has the largest influence over the equilibrium methane concentration exiting the primary reactor, and thus the efficiency.
In a further aspect of the present invention, during those times when steady or continuous liquid water injection is necessary to maintain a constant, or optimum S/C ratio, or a greater volume of liquid water is injected than the superheated flow of steam has the heat capacity to superheat, increasing the heat output of the primary reactor, by its exothermic reaction, can provide additional heat input in the primary reactor to replace heat lost by the continuous injection of relatively colder liquid water. In this aspect, the primary reactor heat load, and the O/C ratio can be controlled by adjusting the air flow rate to the primary reactor. Increasing the air flow rate increases the partial oxidation dissociation rate of the primary reactor, an exothermic reaction, thereby increasing the heat produced by the primary reactor, which can be used to superheat an increased steam volume and further improve reformate gas production.
In this further aspect of the invention, a method of operating a primary reactor in a fuel processing system over a semi-continuous time period to produce a hydrogen-containing reformate from a hydrocarbon fuel is provided, the method comprising the steps of supplying a first stream having a superheated steam to an inlet of the primary reactor; injecting a second stream having liquid water into said first stream; adding a third stream having a preheated fuel to the primary reactor inlet, for dissociation in the primary reactor; introducing a fourth stream having pressurized air to the primary reactor inlet; determining at least one threshold of liquid water volumetric flow above which an increased reaction heat of the primary reactor is required; and controlling a flow rate of the pressurized air to provide the increased reaction heat of the primary reactor.
In yet another aspect of the invention, a superheated air injection rate will be based on the inlet temperature of the primary reactor. Normally, a liquid water injection rate, provided to supplement a transient operation, will decrease gradually over time as the vaporizer heat lag diminishes. If measurement of the primary reactor inlet temperature indicates additional heat input in the primary reactor is required, superheated air can be added to the flowstream upstream of the primary reactor to increase the dissociation rate (an exothermic reaction) of the fuel source, thus increasing the heat load of the primary reactor.
In this aspect of the invention, a method of operating a primary reactor in a fuel processing system to produce a hydrogen containing reformate from a hydrocarbon fuel is provided, the method comprising the steps of supplying a first stream having a superheated steam to an inlet of the primary reactor; injecting a second stream having liquid water into said first stream, during a predetermined operating condition, to vaporize at least a majority of the liquid water in a liquid water/superheated steam mixture prior to the mixture reaching the primary reactor inlet; adding a third stream having a preheated fuel to the primary reactor inlet, for dissociation in the primary reactor; introducing a fourth stream having pressurized air to the primary reactor inlet; measuring an inlet temperature of the primary reactor; and controlling a net flow to the primary reactor, said net flow being a combination of the first, second, third and fourth streams to the primary reactor, based on the measured inlet temperature of the primary reactor.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.